1. Field of the Invention
The present invention relates to an electrophotographic photoconductor, and to an electrophotographic image forming apparatus such as copiers, facsimiles and printers, which include a photoconductor as an image carrier.
2. Discussion of the Background
Inorganic photoconductive materials such as selenium, cadmium sulfide and zinc oxide have been used for electrophotographic photoconductors. However, these materials have drawbacks such as having low photosensitivity and low heat stability, and being toxic. Therefore, currently organic photoconductors have been actively developed, and organic photoconductors having a photoconductive layer including a charge generating material and a charge transporting material are now in practical use in the market.
On the other hand, electrophotographic image forming apparatus such as laser printers and digital copiers, which use a laser diode as a light source, have been developed and practically used in addition to the current image forming apparatus. In order to allow a photoconductor to be commonly used for such various image forming apparatus, the photoconductor is required to have high photosensitivities over a broad wavelength range including the visible region and the near infrared region.
In attempting to develop such a photoconductor, methods in which two or more pigments each of which has a photosensitivity to a wavelength range different from those of the other pigments are used as a charge generating material have been proposed in, for example, Japanese Laid-Open Patent Publications Nos. 63-148264, 1-177553 and 1-270060.
When two or more pigments are used as a charge generating material, the range through which the resultant photoconductor has good photosensitivity widens. However, two or more energy levels are formed in the resultant charge generating layer, and therefore a combination of the characteristics of the pigments cannot be exhibited. Even when the formulation of the photoconductive layer is changed, it is difficult to obtain a photoconductor exhibiting an excellent combination of charge properties including high surface potential and low residual potential.
As to the light source used for image forming apparatus, laser diodes are typically used because of having advantages such as being small in size, low-priced, and easy to handle. The wavelength of the laser light emitted from the marketed laser diodes is limited to the near infrared region not less than 750 nm. Therefore, photoconductors used for these image forming apparatus are required to have photosensitivity over a wavelength range of from 750 to 850 nm.
Squarilium pigments, phthalocyanine pigments, eutectic complexes of a pyrylium dye and a polycarbonate, pyrrolopyrrole, azo pigments and the like are known as the organic photoconductive materials having the requisite properties mentioned above. Among these pigments, phthalocyanine pigments are actively developed for electrophotographic photoconductors because the pigments have absorption and photosensitivity over a relatively long wavelength region, and in addition, by changing the center metal and the crystal form of the phthalocyanine pigments, various kind of photoconductive materials can be prepared.
Up to now, an .epsilon.-type copper phthalocyanine pigment, an X-type metal-free phthalocyanine pigment, a .tau.-type metal-free phthalocyanine pigment, vanadyl phthalocyanine pigment and titanyl phthalocyanine are known as a phthalocyanine pigment having good photosensitivity. However, these phthalocyanine pigments are not satisfactory in the point of photosensitivity, charging ability and durability. Therefore phthalocyanine pigments which are improved in these properties are especially desired.
In Japanese Laid-Open Patent Publication No. 9-127711, it is attempted to solve the problems concerning charge properties by using an azo compound in combination with a phthalocyanine compound. However, concerning the image qualities such as black spots, the publication refers to only the initial image properties, and the resultant photoconductor still has a problem in that image qualities deteriorate when the images are repeatedly produced for a long time.
In addition, Japanese Laid-Open Patent Publications Nos. 7-128890 and 8-29998 have disclosed a combination of a metal-free phthalocyanine pigment with an asymmetric disazo pigment. The purpose of the invention is to attain panchromatic sensitivity and high sensitivity, and the improvement of durability in the properties such as charge properties, image qualities and adhering properties of the photoconductive layer, which is discussed in the present application is not described, or is insufficiently described therein. Therefore, the problems have not been satisfactorily improved.
Further, when a photoconductor provided in an image forming apparatus is often exposed to light (particularly to ultraviolet light) in such a case that a photoconductor unit or developer is changed or a jammed sheet is removed from the apparatus, a problem which occurs is that the charge properties of the photoconductor tends to deteriorate. This problem has not been improved.
Because of these reasons, a need exists for a photoconductor which has stable charge properties and can produce images having good image qualities even when repeatedly used and even after the photoconductor is exposed to light such as ultraviolet light.